The present invention relates generally to beam shearing systems and their applications, particularly in spatially modulated or static interferometers, which have low volume, low mass, high spectra: resolution, a single instrument line shape function and are field widened.
Interferometers are a class of instruments that convert light from a source into an interference fringe pattern or interferogram. Interferometers make measurements on light within a certain portion of the spectrum. This portion is referred to as the predetermined spectral passband of the interferometer. Frequencies of light outside of the predetermined spectral passband are attenuated so that they do not cause inaccuracies in measurements made by the interferometer.
Spatially modulated or static interferometers use a beam shearing system to shear the input beam into two separate beams and a Fourier optical system can be used to recombine the two sheared beams at a detector array or photographic plate. This procedure generates an optical path difference across the wavefronts of the recombined beams, which results in the formation of a modulation pattern that is fixed in space. If a two dimensional detector array is used, then it can record the spatially modulated pattern as an interferogram in one dimension of the detector array and an image in the orthogonal direction.
Static interferometers are distinguished from other types of interferometers in that they do not require the movement of an optical component or the observational platform to generate an interferogram. Interferometers that require the movement of the observing platform or an optical element to generate their spectrum over a given time interval are prone to unrecoverable spectral errors.
The first reported static interferometer was a Michelson interferometer with tilted mirrors built by G. Stroke and A. Funkhouser (see G. W. Stroke et al., Physics Letters, 1965, Volume 16, Number 3, Pg. 272). Two decades later, T. Okamoto et al used a triangular Sagnac interferometer and a conventional camera lens to generate an interferogram (see T. Okamoto et al., Applied Optics, 1984, Volume 23, Number 2, Pg. 269). In all reported cases, traditional interferometer configurations and conventional Fourier Lenses have been used to generate the interferograms, and the detector arrays have operated in the ultra-violet to near infrared wavelength regimes.
The ability to operate a static interferometer at longer wavelengths than those used by existing static interferometers would offer certain advantages. For instance, operation in the thermal-infrared spectrum would enable fewer pixels to be used to sample the fringes of an interferogram because the frequency of fringes decreases with increased wavelength. Another advantage of operating in the thermal-infrared spectrum is that the surfaces of the optical components would no be required to meet the stringent surface quality and accuracy requirements of shorter wavelengths interferometers in order to prevent the generation of surface induced fringes that introduce errors by canceling the interferogram fringes. Thus, if a static interferometer were constructed that could operate in the thermal-infrared, then its optical components would be less costly and less time consuming to manufacture.
Conventional interferometer configurations, such as the Michelson and Sagnac interferometers, utilize beam shearing systems that waste at least 50% of the signal. These configurations typically require the beam of light input into the system to make two passes through a beam splitter during the shearing process. Light is therefore reflected back out the entrance through which it entered, resulting in the loss of at least one half of the light entering the static interferometer.
An alternative method that does not rely on the use of a beam splitter to generate a difference in optical path length was described by Padgett et al. U.S. Pat. No. 5,781,293. This method involves polarizing the input bean and then shearing it using birefringent crystals. Despite elimination of the beam splitter, at least 50% of the light entering this type of system is lost due to absorption by the input polariser.
A particular advantage of the Padgett et al static interferometer over other conventional interferometers is that it is field widened. Being field widened means that the slit can be increased to any reasonable width without influencing the spectral resolution. An interferometer will be field widened when it records the interferogram at a pupil plane. At a pupil plane, diffraction does not degrade spectral resolution.
A disadvantage of existing static interferometers is that their physical volume and mass increase significantly when high spectral resolution is required. This greatly increases cost in applications such as remote sensing devices mounted on satellites or space exploration vehicles.
Another disadvantage is that existing static interferometers do not have a single instrument line shape. The instrument line shape is the characteristic shape of the spectrum generated by the static interferometer when the instrument observes a particular frequency of radiation that is substantially narrower in bandwidth than the spectral resolution of the instrument. In existing static interferometers the line shape changes depending on the frequency of the radiation observed. These instruments must be calibrated for the line shape of each frequency in the instrument""s bandwidth, which is a time consuming process. Data collected by these static interferometers are difficult to analyze and they are not suitable for generating high spectral resolution output in real time. It is desirable to use a static interferometer that possesses a single instrument line shape and which has near perfect spectral registration. This means that the detector array""s output has a single line shape and the lines for the different frequencies are evenly spaced along the spectrum at equal wavenumber intervals. This simplifies the time required to calibrate the instrument and the time required to analyze the data recorded by the instrument.
Accordingly, it is desirable to develop a new static interferometer that is compact, makes use of the majority of incoming radiation, is field widened, can operate in the thermal-infrared region of the spectrum, has a single instrument line shape and has near perfect spectral registration.
In one aspect, the static interferometer of the present invention is capable of providing an instantaneous single-sided interferogram in a tangential exit pupil plane and an image in a sagiital image plane, both of which are located at the same point along the optical axis. The instrument can have an optical efficiency approaching 100 percent, has a high signal-to-noise ratio and is field widened. Because the interferogram is generated at a pupil plane by two perfectly collimated beams, the interferogram is not affected by diffraction. This characteristic enables the instrument to possess spectral radiometric purity, have a very broad spectral bandwidth and have the ability to operate within the thermal-infrared spectrum. In addition, this characteristic enables it to have a single instrument line shape and near-perfect spectral registration. Finally, the instrument is a compact and lightweight unit that is easy to align during construction and simple to calibrate.
In one particularly advantageous embodiment, the fore-optics collect light and focus it onto an entrance slit. The light passes through the entrance slit and into the beam shearing system, which splits it into two separate beams. The beam shearing system is constructed to ensure that the two beams of light emerging from it contain more than 50 percent of the collected light that is within the predetermined spectral passband of the instrument. The emerging beams are incident on a Fourier optical system, which collimates and recombines them onto the exit pupil plane. The recombined beams of light generate an interferogram on a detector line array located at a tangential exit pupil plane, enabling the intensity of the interferogram to be measured by the detector, read out by electronics and then digitized by an analogue to digital converter. The data processing system then manipulates the digital data to extract useful information concerning the spectral composition of the collected light. When fore-optics with a shifted pupil are used, measurements can be made using a single sided interferogram at the tangential exit plane. When a Fourier optical system is used that also focuses the light onto a sagiital image plane located at the same point on the optical axis as the tangential exit pupil plane, then a two-dimensional detector array can then be used to record the intensities of both the image and the interferogram.
The forgoing results are preferably achieved by static interferometers having: fore-optics for collecting light and focusing it into a beam; a spectral resolving system comprising of a beam shearing system to split :he beam of light having a photon flux within a predetermined spectral passband, an optical system for recombining the two split beams onto an exit pupil, and a detector located at the exit pupil. The beam shearing system preferably includes: an entrance slit structure having an entrance slit extending in a first direction for receiving the light collected by the fore-optics; a beam splitter aligned at an angle to the first direction so that the received beam of light is split into two separate beams; a reflective subsystem having a plurality of reflective surfaces defining separate light paths of equal optical path length for the two separate beams, the reflective surfaces arranged such that the two beams contain more than 50 percent of the photon flux that is within the predetermined spectral passband of the collected light. In this embodiment, the chief rays of the two separate beams are also substantially parallel to each other and the two light paths are of substantially equal optical path length.
In one form, the reflective surfaces are also arranged to ensure that the two beams remain substantially in phase relative to one another. In another form, a fore-optics may to have a shifted pupil design to generate single-sided interferograms at the exit pupil plane. In yet another form, the optical system has an optical axis and also recombines the beams that emerge from the beam shearing system to create a sagiital image plane located at the same point along that optical axis as the tangential exit pupil plane. In yet another form again, the interferometer contains a detector array, read out electronics and data processing system. The detector array records the intensity of the radiation incident on its pixels, the read out electronics digitizes these measurements and transfers them to the data processing system, and the data processing system manipulates the digitized measurements to obtain information about the spectrum of the incident radiation. In a still further form, the data processing system performs Fast Fourier Transforms (FFTs) on the digitized data to obtain the spectrum of the collected light. In a still further form again, the data processing system convolves the digitized data with digital filters to detect the presence or absence in the spectrum of the collected light of frequencies characteristically emitted or absorbed by particular chemicals.